Design specification for smart speakers v1.0

Preface

This specification is based on the author's current work experience , If there is any deficiency , Welcome to correct .

1. Microphone system design

1.1 MIC Model selection

1. Smart speakers are right MIC High array consistency requirements , Due to the production process , simulation MIC Poor consistency , Therefore, numbers are preferred MIC;
2. simulation MIC Its advantage is high signal-to-noise ratio , And the gain is adjustable , Under the condition of ensuring consistency , simulation MIC Have more advantages ;
3. Sensitivity requirements ： Greater than [email protected] 1kHz;
4. Signal to noise ratio requirements ： Greater than 64dB;
5. Acoustic overload point （AOP） requirement ： Greater than 120dBSPL
6. MIC Monomer distortion requirements ： Less than 1%@1kHz

1.2 MIC Sound outlet design

1. The microphone hole on the housing should be larger than 0.8mm;
2. The smaller the volume of the front cavity, the better , The shorter the hole length on the shell, the better , The larger the diameter of the hole, the better , Generally, the diameter depth ratio of the sound outlet is not less than 1:3;
3. It can be completed with independent integrated structural parts MIC seal up & Construction of radio channel , Do not use multiple assemblies , The error chain in production will lead to the decrease of sealing consistency ;
4. The dust screen can be selected with appropriate density and sound resistance , Choosing a dust net will increase the cost ;
5. Bottom sound MIC The sealing structure is easy to do ;
6. MIC The installation position shall be as far away from the noise source as possible （ Such as motor 、 Fans, etc ）;
7. The sound outlet shall avoid the occurrence of the resonant cavity （ Small at both ends , Big in the middle ）;

1.3 MIC Formation selection

1. MIC The more the number of , The better the result. , But the higher the cost ;
2. MIC The formation is divided into ring array and linear array , The ring array shall be placed at equal intervals as far as possible MIC, Array diameter range 60~80mm, It is widely used in the industry 70mm;
3. For example, the structure of unrestricted priority ring array . Affected by the array beam , The angle identification deviation of linear array in the axis direction is large .

2. Loudspeaker system design

2.1 Speaker unit selection

1. Distortion of loudspeaker unit （THD） As small as possible , The less distortion , Echo cancellation （AEC） The better the result. , The higher the interrupt wake-up rate ;
2. When the power is satisfied , Ferrite speakers are preferred , The cost is low , Ferrite also acts as a counterweight ;
3. The maximum volume of the system shall not exceed the rated power of the loudspeaker unit ;

2.2 Loudspeaker structure design

1. If the structure permits , The speaker and microphone should be as far away as possible ;
2. The loudspeaker and microphone shall be in different cavities , Seal well , Prevent internal transmission ;
3. The loudspeaker system shall be subject to shock absorption treatment , Prevent structural vibration from causing structural sound transmission , And keep a certain distance from other components , Prevent abnormal signal caused by collision ;

3. Circuit system design

3.1 Mining circuit design

1. According to different mining positions, it can be divided into internal mining and external mining , The external recovery is divided into the recovery before the external power amplifier and the recovery after the external power amplifier , The recommended mining type is mining after external power amplifier , That is, the recovery is located between the power amplifier and the loudspeaker （ After the power amplifier , Before the loudspeaker , such AEC The best effect ）;
2. If there is automatic gain control in the recovery circuit （AGC）, It should be placed before the stoping position ;

3.2 Signal quality requirements

1. Power matching ： The device plays the full sweep signal at its maximum volume ,MIC The signal cannot be truncated ;
2. Self broadcast and self recorded audio signal ,MIC The signal shall be later than the recovery signal , It cannot be earlier than the recovery signal ;
3. Requirements for signal-to-noise ratio of the whole machine ： Not less than [email protected] 1kHz;
4. Sensitivity requirements of the whole machine ： Not less than [email protected] 1kHz;
5. MIC Too small signal will cause the far-field wake-up rate to decrease , When the device plays a full sweep signal at its maximum volume , The maximum amplitude range is -3dBFS~-9dBFS It is advisable to ;